Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor either directly (“directly driven”) or through the use of a gearbox.
A variable speed wind turbine may typically be controlled by varying the generator torque and the pitch angle of the blades. As a result, aerodynamic torque, rotor speed and electrical power generated will vary.
A common prior art control strategy of a variable speed wind turbine may be described with reference to FIG. 1a. In FIG. 1a, the operation of a typical variable speed wind turbine is illustrated in terms of the pitch angle (β), the electrical power generated (P), the generator torque (M) and the rotational velocity of the rotor (ω), as a function of the wind speed.
In a first operational range, from the cut-in wind speed to a first wind speed (e.g. approximately 5 or 6 m/s), the rotor may be controlled to rotate at a substantially constant speed that is just high enough to be able to accurately control it. The cut-in wind speed may be e.g. approximately 3 m/s.
In a second operational range, from the first wind speed (e.g. approximately 5 or 6 m/s) to a second wind speed (e.g. approximately 8.5 m/s), the objective may generally be to maximize power output while maintaining the pitch angle of the blades so as to capture maximum energy. In general, in the second operational range, the pitch angle of the blades may be substantially constant, although the optimal blade setting may theoretically depend on the instantaneous wind speed. In order to achieve this objective, the generator torque and rotor speed may be varied so as to keep the tip speed ratio A (tangential velocity of the tip of the rotor blades divided by the prevailing wind speed) constant so as to maximize the power coefficient Cp.
In order to maximize power output and keep Cp constant at its maximum value, the rotor torque may be set in accordance with the following equation:T=k·ω2,wherein k is a constant, and ω is the rotational speed of the generator. In a direct drive wind turbine, the generator speed substantially equals the rotor speed. In a wind turbine comprising a gearbox, normally, a substantially constant ratio exists between the rotor speed and the generator speed.
In a third operational range, which starts at reaching nominal rotor rotational speed and extends until reaching nominal power, the rotor speed may be kept constant, and the generator torque may be varied to such effect. In terms of wind speeds, this third operational range extends substantially from the second wind speed to the nominal wind speed e.g. from approximately 8.5 m/s to approximately 11 m/s.
In a fourth operational range, which in some cases may extend from the nominal wind speed to the cut-out wind speed (for example from approximately 11 m/s to 25 m/s), the blades may be rotated (“pitched”) to maintain the aerodynamic torque delivered by the rotor substantially constant. In practice, the pitch may be actuated such as to maintain the rotor speed substantially constant. At the cut-out wind speed, the wind turbine's operation is interrupted.
In the first, second and third operational ranges, i.e. at wind speeds below the nominal wind speed (the sub-nominal zone of operation), the blades are normally kept in a constant pitch position, namely the “below rated pitch position”. Said default pitch position may generally be close to a 0° pitch angle. The exact pitch angle in “below rated” conditions however depends on the complete design of the wind turbine.
The before described operation may be translated into a so-called power curve, such as the one shown in FIG. 1a. Such a power curve may reflect the optimum operation of the wind turbine under steady-state conditions and under conditions of uniform wind speed over the rotor swept area (the area swept by the blades of the wind turbine).
If the wind is not uniform over the swept area and/or if the wind is variable, a steady state power such as the one depicted in FIG. 1a (and its accompanying control strategy) does not necessarily lead to optimum operation of the wind turbine.
A first possible non-uniformity over the rotor plane is the phenomenon of wind shear. With reference to FIG. 1b, the phenomenon of wind shear may be explained. Wind shear is a variation of wind speed with height. A wind turbine comprising a tower 5 carrying a rotor with blades 1, 2 and a third non-visible blade is illustrated in FIG. 1b. The tower 5 has height h. At height h, the wind speed is Vh. This wind speed may be measured e.g. by an anemometer based on the nacelle.
This however does not mean that the wind speed is constant over the entire rotor swept area. The wind speed may vary in accordance with wind profile 52. In particular, at increased heights, the wind speed may be higher, such as indicated in FIG. 1b, and the wind speed may be lower at heights below the nacelle. The resulting wind speed, i.e. the wind speed that would be more representative for the wind energy as perceived by the whole of the rotor may be e.g. the wind speed indicated by reference sign 51.
Although in this example a “classic” example of wind shear has been illustrated with a wind speed increasing with height, this does not necessarily need to be the case for all wind turbine sites. Depending on the site, e.g. phenomena of “low level jets” may be experienced. Low level jets may lead to the situation that e.g. at hub height the wind speed is significantly higher than at the top of the rotor swept area.
Also in the case of wind veer, the phenomenon of varying wind direction with height may lead to suboptimum results based on a fixed power curve to steady state conditions (illustrated in FIG. 1c).
It is known e.g. from EP 2 607 689 to use a LIDAR system to measure wind speeds in a plane upstream from a rotor plane of the wind turbine. According to EP 2 607 289 a control system for individually pitching the blades relative to the hub is provided. The rotor plane is divided into a plurality of predefined sections, wherein each section has an associated pitch reference value. A LIDAR may be used to determine expected properties of wind in each respective section of the rotor plane so that the pitch reference value may be adjusted accordingly.
One disadvantage related to such prior art systems is firstly that a rather expensive LIDAR system is needed for every wind turbine. Another disadvantage is that a quick forward feed control system and quick actuation system needs to be employed to take advantage of the information provided by the LIDAR. Constantly pitching the blades to adapt to gusts, wind shear, wind veer, turbulence and other phenomena for individual sections of the rotor swept area may lead to excessive wear on the pitch system.
The present disclosure relates to various methods and systems for avoiding or at least partly reducing one or more of the aforementioned problems.